1. Field of the Invention
The present invention relates to a method for cleaning the surface of a partially manufactured integrated circuit just prior to the deposition of a metal layer. A particular feature of the method is that it eliminates subsequent peeling of the metal layer without introducing a high electrical contact resistance between the metal layer and the integrated circuit.
2. Description of the Prior Art
It has long been recognized in the manufacture of integrated circuits that as successive layers of material are deposited it is essential that each surface that is about to receive a fresh deposit be absolutely clean. A particular problem is that various liquids that come in contact with these surfaces, as part of the routine processing needed for the creation of integrated circuits, may, in some instances, leave insoluble and often invisible residues behind. These could be the result of trace impurities in the liquids or they could be the byproducts of chemical side reactions that occurred during processing.
If appropriate measures are not taken to completely remove these residues, a subsequently deposited layer will not adhere well to the surface and will be subject to peeling, blistering, etc. Thus, a number of procedures have been developed for cleaning the surface of an integrated circuit just prior to film deposition. An example of such a cleaning procedure is the one described by Kern and Puotin (RCA Review P. 187 June 1970):
1) Any residual photoresist is removed by immersion in an inorganic resist stripper such as a mixture of Sulphuric acid (H.sub.2 SO.sub.4) and Hydrogen Peroxide (H.sub.2 O.sub.2). Wafers are then rinsed in 18-20 megohm-cm. deionized water.
2) This is followed by immersion in a mixture of water, Ammonium Hydroxide (NH.sub.4 OH), and H.sub.2 O.sub.2 (5:1:1 by volume) for 10-15 minutes at a temperature of 75-80.degree. C. and then rinsed as before.
3) The next step is a brief dip in Hydrofluoric Acid (diluted with water 1:10) followed by a brief rinse.
4) Then, wafers are immersed in a mixture of water, hydrochloric acid (HCl), and H.sub.2 O.sub.2 (6:1:1 by volume) for 10-15 minutes at a temperature of 75.degree.-80.degree. C. followed by a rinse, as before.
5) Finally, wafers are dried by heating in nitrogen and are then ready to receive the next deposited layer.
While the method that has been achieved in this invention bears some resemblance to the above, it differs in several of the details and, more particularly, as will be seen when the experimental data is presented below, it teaches that the order in which certain of the steps is performed is crucial, as well as different from that described above.
Quite a different approach to the cleaning of surfaces prior to film deposition is sputter etching. In this method a very thin layer (typically 1-200 Angstrom Units) is removed by sputtering. Since the contaminating residue layers are typically thinner than this, their removal is guaranteed, along with a small amount of the underlying surface. There are two problems associated with this method. It can be difficult to control so there is a tendency to overetch which can lead to serious problems. Additionally, recontamination of the surface can occur as a result of backscattering of some of the sputtered material. It has been reported by, for example, Vossen (J. Vac. Sci. & Tech. vol. A2 no. 2 1984 p. 212) that such backscattering can lead to increased contact resistance between freshly deposited films and previously deposited ones.
A particular feature of the method of this invention is that it does not lead to an increase in the contact resistance between successively deposited films.